#include <bits/stdc++.h>
using namespace std;

class Solution {
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {-1, 1, 0, 0};
    int m, n;

public:
    void solve(vector<vector<char>>& board) {
        m = board.size(), n = board[0].size();
        for (int j = 0; j < n; j++)
            dfs(board, 0, j);
        for (int i = 1; i < m - 1; i++)
            dfs(board, i, 0);
        for (int j = 0; j < n; j++)
            dfs(board, m - 1, j);
        for (int i = 1; i < m - 1; i++)
            dfs(board, i, n - 1);

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'O')
                    board[i][j] = 'X';
                else if (board[i][j] == '.')
                    board[i][j] = 'O';
            }
        }
    }
    void dfs(vector<vector<char>>& board, int i, int j) {
        if (board[i][j] == 'O') {
            board[i][j] = '.';
            for (int k = 0; k < 4; k++) {
                int x = dx[k] + i;
                int y = dy[k] + j;
                if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O') {
                    dfs(board, x, y);
                }
            }
        }
    }
};